I don't have access to a windows 10 system as of now. It should work fine, though. If it doesn't, I may need to rewrite the installer, because nothing, technically, would prevent HighC from working on Windows 10.

The only thing that may create problem is that OS providers are more and more paranoid and restrictive about independent software, and they create all sorts of hurdles (to have us pay them) to small software creators. So, they may ask for HighC to register a certificate or something like that in order to accept that it runs on new versions of the OS.

My question was related to the free upgrade from Windows 8.1 to 10 (which will be instaled without pc format - so it will be not needed reinstallation of the already in 8.1 instaled programs). I subscribed to the expectant list, but also I am "buttoned up" because I am afraid the probability after upgrade some programs may be not work.

I think you can safely upgrade to 10. If HighC doesn't work after the update, then tell me and I'll search for a quick fix (quite confident there will be one) once you describe the problem, if there is one indeed.